Cannot instantiate ObservableList with an extractor

I have a custom object FermentableInRecipe, which populates a TableView. In order to respond to changes to items in the list, as well as the list itself, I have decided to employ an extractor. Here is my declaration and instantiation of my ObservableList:

private ObservableList<FermentableInRecipe> fermentablesInRecipe = 
FXCollections.observableArrayList(item -> new Observable[]{item.WeightProperty()});

Here are the relevant segments of my custom class:

public class FermentableInRecipe {

    private DoubleProperty weight;

    ...

    public Double getWeight() {
         return this.weight.getValue();
    }

    public void setWeight(Double value) {
        this.weight.setValue(value);
    }

    public DoubleProperty WeightProperty() {
        if (weight == null) {
            weight = new SimpleDoubleProperty(0.0);
        }
        return weight;
    }

    ...
}

In the links I've provided below, this approach worked. But Netbeans is telling me "DoubleProperty cannot be converted to Observable". I can see why this is the case, but I cannot understand why it worked in the links below and not for me, and how I should create extractor and link it to the weightProperty() function if this approach doesn't work.

Links:

JavaFX 2.0 Choice Box Issue. How to update a choiceBox, which represents a list of objects, when an object is updated?

JavaFX, ObservableList: How to fire an InvalidationListener whenever an object of the list gets modified?

Thanks in advance. Let me know if I've missed any crucial information.

There's nothing wrong with your code as written, this compiles just fine for me:

import javafx.application.Application;
import javafx.beans.Observable;
import javafx.beans.property.DoubleProperty;
import javafx.beans.property.SimpleDoubleProperty;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.stage.Stage;

public class JavaFXApplication1 extends Application {

    class FermentableInRecipe {

        private DoubleProperty weight;

        public Double getWeight() {
            return this.weight.getValue();
        }

        public void setWeight(Double value) {
            this.weight.setValue(value);
        }

        public DoubleProperty WeightProperty() {
            if (weight == null) {
                weight = new SimpleDoubleProperty(0.0);
            }
            return weight;
        }

    }

    private ObservableList<FermentableInRecipe> fermentablesInRecipe = FXCollections.observableArrayList(item -> new Observable[]{item.WeightProperty()});

    @Override
    public void start(Stage primaryStage) throws Exception {
    }
}

I'd suggest double checking imports, and make sure you haven't imported java.util.Observable or similar by mistake.